Feedback scheme for mu-mimo

ABSTRACT

Embodiments of a system and method for managing feedback in a MU-MIMO system. An access point can announce one or more of mobile stations that are to receive downlink information in a first frame. The access point can also send a sounding package to the one or more mobile stations and receive feedback from the one or more mobile stations according to the feedback schedule. The feedback may be based on the reception of the sounding package.

BACKGROUND

Multiple-user multiple-input multiple-output (MU-MIMO) systems cantransmit and receive signals to/from multiple users at a single antennaarray at the same time. In a MU-MIMO system, multiple signals are sentin parallel and are kept separate from one another by transmitting (orreceiving) each signal in a different (e.g., orthogonal) direction or ina different spatial channel. The process of transmitting (or receiving)a signal in a specific direction using an antenna array is known asbeamforming. By selecting a beam for each signal that will producelimited interference with other parallel beams, multiple signals can betransmitted or received at the same time. Beams are often calculated tobe orthogonal to one another in order to minimize the interferencebetween the beams.

There are two different types of beamforming systems: implicit andexplicit. In implicit beamforming the access point calculates thebeamforming matrix based on reciprocal channels. That is, the accesspoint receives uplink signals and based on the reception quality anddirection of the uplink signals, the access point determines thedirection and other parameters for the downlink beams. In explicitbeamforming the access point sends out a sounding signal to the mobilestations. The mobile stations then send feedback to the access pointregarding the quality of the received signal. The access point can thenuse the feedback to generate beams for future downlink signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a wireless communication system.

FIG. 2 illustrates an example of a mobile station for communicating inthe wireless communication system of FIG. 1.

FIG. 3 illustrates an example of an access point for communicating inthe wireless communication system of FIG. 1.

FIG. 4 illustrates an example chart for managing feedback for explicitbeamforming in the system of FIG. 1

DETAILED DESCRIPTION

The following description and the drawings sufficiently illustratespecific embodiments to enable those skilled in the art to practicethem. Other embodiments may incorporate structural, logical, electrical,process, and other changes. Portions and features of some embodimentsmay be included in, or substituted for, those of other embodiments.Embodiments set forth in the claims encompass all available equivalentsof those claims.

The present inventors have recognized, among other things, a feedbackscheme for explicit beamforming enabling multiple mobile stations toprovide feedback to an access point in a MU-MIMO system. In the feedbackscheme the access point allocates uplink time slots to each mobilestation for feedback information. In order to allocate the uplink timeslots, the access point determines the length of the feedback from eachmobile station. In some examples, the access point can control thelength of the feedback from the mobile stations by assigning a number ofdownlink streams to each mobile station as well as limiting the feedbackto a specific type, and a specific modulation and coding scheme. As usedherein a stream corresponds to a stream of data to be transmitted as abeam using beamforming techniques and/or spatial multiplexing (SDMA)techniques.

FIG. 1 illustrates an example of a wireless communication system 100.The wireless communication system 100 can include a plurality of mobilestations 102 in wireless communication with an access network 104. Theaccess network 104 forwards information between the mobile stations 102and another communications network 106. Communications network 106 caninclude the internet, a private intranet, or other network.

In an example, each mobile station 102 can include one or more antennas114 for transmitting and receiving wireless signals to/from an accesspoint 118 in the access network 104. The access point 118 can implementthe air interface to the mobile stations 102, and can transmit andreceive signals with an antenna array 116 coupled thereto. The accesspoint 118 can be communicatively coupled to the communications network106 for forwarding information to/from the mobile stations 106.

FIG. 2 illustrates an example of a mobile station 102. The mobilestation 102 can include a memory 202 for storage of instructions 204 forexecution on processing circuitry 206. The instructions 204 can comprisesoftware configured to cause the mobile station 102 to perform actionsfor wireless communication between the mobile station 102 and the accesspoint 118. The mobile station 102 can also include an RF transceiver 208for transmission and reception of signals with the antenna 114.

In some examples, the mobile station 102 can be a personal digitalassistant (PDA), a laptop or desktop computer with wirelesscommunication capability, a web tablet, a net-book, a wirelesstelephone, a wireless headset, a pager, an instant messaging device, adigital camera, an access point, a television, a medical device (e.g., aheart rate monitor, a blood pressure monitor, etc.), or other devicethat can receive and/or transmit information wirelessly.

FIG. 3 illustrates an example of an access point 118. The access point118 can include a memory 302 for storage of instructions 304 forexecution on processing circuitry 306. The instructions 304 can comprisesoftware configured to cause the access point 118 to perform actions forwireless communication between the mobile station 102 and the accesspoint 118. The access point 118 can also include an RF transceiver 308for transmission and reception of signals using the antenna array 116.The processing circuitry 306 can be configured to implement beamformingwith the antenna array 116. In an example, the processing circuitry 306can be configured to use the antenna array 116 to implement adaptivebeamforming in a MU-MIMO system. That is, multiple beams can beimplemented at the same time to different mobile stations 102. Moreover,the direction of each beam can change dynamically according to changesin the signal path to a given mobile station 102. The access point 118can include a network switch, router, or hub for sending and receivinginformation with the communications network 106.

In an example, the mobile station 102 and access point 118 can beconfigured to operate in accordance with one or more frequency bandsand/or standards profiles. For example, the mobile station 102 andaccess point 118 can be configured to communicate in accordance withspecific communication standards, such as the Institute of Electricaland Electronics Engineers (IEEE) standards. In particular, the mobilestation 102 can be configured to operate in accordance with one or moreversions of the IEEE 802.11ac communication standard for MU-MIMO Wi-Fi.

The mobile station 102 and access point 118 can also be configured tooperate in accordance with one or more versions of the IEEE 802.16communication standard for wireless metropolitan area networks (WMANs)including variations and evolutions thereof. For example, the mobilestation 102 and access point 118 can be configured to communicate usingthe IEEE 802.16-2004, the IEEE 802.16(e), and/or the 802.16(m) versionsof the 802.16 standard. For more information with respect to the IEEE802.16 standards, please refer to “IEEE Standards for InformationTechnology—Telecommunications and Information Exchange betweenSystems”—Metropolitan Area Networks—Specific Requirements—Part 16: “AirInterface for Fixed Broadband Wireless Access Systems,” May 2005 andrelated amendments/versions.

In some examples, the mobile station 102 and access point 118 can beconfigured to communicate in accordance with one or more versions of theUniversal Terrestrial Radio Access Network (UTRAN) Long Term Evolution(LTE) communication standards, including LTE release 8, LTE release 9,and future releases. For more information with respect to UTRAN LTEstandards, see the 3rd Generation Partnership Project (3GPP) standardsfor UTRAN-LTE, release 8, March 2008, including variations and laterversions (releases) thereof.

In some examples, RF transceiver 208 and RF transceiver 308 can beconfigured to transmit and receive orthogonal frequency divisionmultiplexed (OFDM) communication signals which comprise a plurality oforthogonal subcarriers. In broadband multicarrier examples, the mobilestation 102 and access point 118 can be configured to communicate inaccordance with an orthogonal frequency division multiple access (OFDMA)technique.

In other examples, the mobile station 102 and access point 118 can beconfigured to communicate using one or more other modulation techniquessuch as spread spectrum modulation (e.g., direct sequence code divisionmultiple access (DS-CDMA) and/or frequency hopping code divisionmultiple access (FH-CDMA)), time-division multiplexing (TDM) modulation,and/or frequency-division multiplexing (FDM) modulation.

FIG. 4 illustrates an example timing flowchart 400 for a feedback schemefor explicit beamforming in the MU-MIMO system 100. To start thefeedback process, the access point 118 sends out a sounding package 402to the mobile stations 102. The sounding package 402 can correspond toone or more downlink frames 404. The sounding package 402 can announcethe group ID for the downlink frames 404, as well as the media accesscontrol (MAC) address for the mobile stations 102 to be associated withthe group ID.

The sounding package 402 can also include a feedback schedule for themobile stations 102. The feedback schedule can identify time slots foreach of the mobile stations 102 associated with the Group ID. In anexample, in order to efficiently schedule the feedback time slots, theaccess point 118 determines the length of the feedback from each mobilestation 102 and assigns the time slots accordingly. For example, theaccess point 118 can announce that a first mobile station 102 has afeedback time slot 406 scheduled at the beginning of the feedbackperiod. After determining the length of the feedback from the firstmobile station 102, the access point 118 can assign a second mobilestation 102 to a feedback time slot 408 immediately following the end ofthe feedback 406 from the first mobile station 102. The access point 118can continue assigning feedback time slots 410, 412 in this manner untilall the mobile stations 102 associated with the group ID have a timeslot in the feedback period.

In an example, in order to determine the length of the feedback to bereceived from the mobile stations 102, the access point 118 specifiescertain parameters for the feedback. For example, the length of thefeedback for a mobile station 102 can vary based on the number ofdownlink streams for the mobile station 102 and the dimension of thefeedback matrix received from the mobile station 102.

Accordingly, in an example, the access point 118 specifies the dimensionof the feedback matrix for the mobile station 102. The dimension of thefeedback matrix assigned by the access point 118 can be set to greaterthan or equal to the number of downlink streams on which data will besent to the mobile station 102 in the downlink frame associated with thegroup ID. In an example, the number of streams allocated to a mobilestation 102 is dependent upon the number of different mobile stations102 to receive transmissions within a given downlink frame. When a largenumber of mobile stations 102 are to receive transmissions, there may befewer streams available for use by each mobile station 102. Accordingly,the number of streams allocated to the mobile station 102 may be lessthan if there were fewer mobile stations 102 to receive transmissionswithin the downlink frame. In an example, the access point 118 has amaximum of eight streams and a maximum of four different mobile stations102 per downlink frame. Additionally, in an example, a maximum of fourstreams can be sent to a single mobile station 102. In an example, thedimension of the feedback matrix assigned to a mobile station 102 candepend on the precoding type as well as the number of downlink streams.For example, when the access point 118 implements a simple MMSEprecoding, the dimension of the feedback matrix can be set to equal thenumber of downlink streams (Ns×Nt). When the access point 118 implementsa block diagonal precoding, then the dimension of the feedback matrixcan be set to the number of receive antennas (Nr×Nt). The number ofstreams allocated to a mobile station 102 can also be based on thequality of previous transmissions from the mobile station 102. Forexample, if the previous transmission experienced bad quality due tohigh interference between multiple streams, then the access point 118can reduce the number of streams allocated to the mobile station 102 infuture transmissions.

The access point 118 can also specify the feedback type for the mobilestation 102 to use. In an example, the feedback types includeun-compressed channel state information (CSI) feedback, un-compressedprecoding matrix feedback, and compressed precoding matrix feedback. Theun-compressed CSI feedback can comprise one or more columns of the Nr×Ntchannel matrix, depending on the feedback matrix dimension specified bythe access point 118. As used herein, Nr corresponds to the number ofreceive antennas at the mobile station 102 and Nt corresponds to thenumber of transmit antennas in the antenna array 116 on the access point118. Additionally, Ns corresponds to the number of streams allocated tothe mobile station 102 in the sounding package 402. The un-compressedprecoding matrix has the same dimensions as the un-compressed CSIfeedback, but the content of the matrix is the right singular vector ofthe channel matrix, instead of CSI itself. The compressed precodingmatrix comprises a compressed information representing the rightsingular matrix of the channel matrix. For example, if the access point118 has four transmit antennas in the antenna array 116, the mobilestation 102 has two receive antennas, and the access point 118 indicatesin the sounding package 402 that Ns=2, then the un-compressed CSI andprecoding matrix would comprise a 4×2 matrix. The compressed precodingmatrix would comprise a 4×2 right singular matrix. More compressedfeedback types can also be used. In an example, the mobile station 102notifies the access point 118 of the feedback types that the mobilestation 102 supports. In an example, this notification of feedback typesupport can be provided by the mobile station 102 during associationwith the access point 118.

In an example, the access point 118 can set the feedback type based onthe precoding algorithm used by the access point 118. For example, whenthe access point 118 is using a simple precoding algorithm, the accesspoint 118 can specify a feedback type providing less information fromthe mobile stations 102. When the access point 118 is using a complexprecoding algorithm, the access point 118 can specify a feedback typeproviding more information from the mobile stations 102. For example, ifthe access point 118 is using the simple minimum mean square error(MMSE) precoding algorithm, then the access point 118 can specify thatthe mobile stations 102 send feedback using the compressed precodingmatrix. If the access point implements a block diagonal precodingalgorithm then AP can specify full channel matrix (e.g., un-compressedCSI) from the mobile stations 102. In an example, when the access point118 specifies that the mobile station 102 is to receive fewer streamsthan the number of receive antennas at the mobile station 102. In anexample, the mobile station 102 can provide feedback based on receptionwith certain receive antennas and can receive the corresponding downlinkframe with the same receive antennas.

In an example, the access point 118 can also specify the modulation andcoding scheme (MCS) to be used by the mobile station 102 when encodingthe feedback for sending to the access point 118. In any case, once theaccess point 118 has specified these parameters, the access point 118can determine that the length of the feedback from each mobile station102 and assign time slots for the feedback according to the length ofthe feedback.

When a mobile station 102 receives a sounding package 402, the mobilestation 102 can measure the preamble of the sounding package 402 todetermine the reception quality and direction. The mobile station 102then calculates a feedback matrix of the feedback type (e.g.,un-compressed CSI, un-compressed precoding, or compressed precoding)specified in the sounding package 402 based on the reception quality anddirection.

In an example, the mobile station 102 can calculate the feedback matrixof the reception quality based on no interference from streams for othermobile stations 102. Advantageously, this method of calculating thefeedback matrix can be the same as for single user MIMO which cansimplify the processes at the mobile station 102 since the mobilestation 102 does not use different processes for MU-MIMO and SU-MIMO. Inanother example, the mobile station 102 can calculate the feedbackmatrix based on random and uniform interference from streams for othermobile stations 102. For example, the mobile station 102 can calculatethe number of streams for other mobile stations 102 based on the streamallocations provided by the access point 118 in the sounding package402. The mobile station 102 can then calculate the feedback matrix byassuming an interference signal from all the other streams randomly anduniformly arriving from all directions, where each stream is transmittedwith the same transmit power.

Once the mobile station 102 has calculated the feedback matrix, themobile station 102 sends the feedback matrix to the access point 118within the time slot allocated to the mobile station 102 and using theMCS announced in the sounding package 402. In an example, in addition tothe feedback matrix, the mobile station 102 calculates a preferred MCSfor use by the access point 118 in the downlink streams to the mobilestation 102. The preferred MCS can be calculated based on nointerference from streams to other mobile stations 102, or based onrandom and uniform interference from streams to other mobile stations102.

All the mobile stations 102 associated with the group ID can perform thefeedback matrix calculation according to the announcements in thesounding package 402 and provide the feedback matrix within the timeslot assigned by the sounding package 402.

Once the access point 118 receives the feedback matrixes from the mobilestations 102, the access point 118 can adjust the parameters of theprecoding algorithm based on the feedback matrixes. In an example, formobile stations 102 that calculated the feedback matrix based on nointerference from streams for other mobile stations 102, the accesspoint 118 can adjust the feedback matrix to account for interferencefrom other streams. In another example, the access point 118 canimplement the feedback matrix without adjustment in the precodingalgorithm. Once the access point 118 has adjusted the parameters basedon the feedback matrixes received, the access point 118 can transmit thedownlink frame 414 that corresponds to the group ID in the soundingpackage 402. Accordingly, the downlink frame 414 can be generated usingprecoding adjustments based on the feedback from the mobile stations102. In an example, the access point 118 applies the adjusted precodingbased on the feedback to the training field as well as to data fields(e.g., MAC protocol data units (MPDUs)) within the downlink frame 414.

A mobile station 102 associated with the group ID can then receive thedownlink frame 414 and decode the information therein. The mobilestation 102 can then send an acknowledge (ACK) or negativeacknowledgement (NACK) information 416 on an appropriate channel (e.g.,an ACK channel) to the access point 118 evidencing whether the downlinkframe 414 was accurately received by the mobile station 102. Inaddition, in some examples, the mobile station 102 can perform anadditional calculation to refine the preferred MCS to the used by theaccess point 118 based on the reception quality of the downlink frame414. This refined MCS can be piggybacked with ACK/NACK information 416and sent to the access point 118 on, for example, an ACK channel. Theaccess point 118 can then use the refined MCS for future transmissionsto the mobile station 102. Advantageously, this refined MCS takes intoaccount actual interference from streams to other mobile stations 102,since these other streams were transmitted in the downlink frame 414along with the stream(s) to the mobile station 102 performing thecalculation.

Embodiments may be implemented in one or a combination of hardware,firmware and software. Embodiments may also be implemented asinstructions stored on a computer-readable medium, which may be read andexecuted by at least one processing circuitry to perform the operationsdescribed herein. A computer-readable medium may include any mechanismfor storing in a form readable by a machine (e.g., a computer). Forexample, a computer-readable medium may include read-only memory (ROM),random-access memory (RAM), magnetic disk storage media, optical storagemedia, flash-memory devices, and other storage devices and media.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b)requiring an abstract that will allow the reader to ascertain the natureand gist of the technical disclosure. It is submitted with theunderstanding that it will not be used to limit or interpret the scopeor meaning of the claims. The following claims are hereby incorporatedinto the detailed description, with each claim standing on its own as aseparate embodiment.

1. A method for managing feedback in a multiple user multiple inputmultiple output (MU-MIMO) system, the method comprising: announcing oneor more mobile stations that are to receive downlink information in afirst frame; sending a sounding package to the one or more mobilestations; assigning a feedback schedule for the one or more mobilestations; and receiving feedback from the one or more mobile stationsaccording to the feedback schedule, the feedback based on reception ofthe sounding package.
 2. The method of claim 1, comprising: receivingfeedback capability from the one or more mobile stations; assigning afeedback matrix dimension for the one or more mobile stations; assigninga modulation and coding scheme for the one or more mobile stations;assigning a feedback type for the one or more mobile stations based onthe feedback capability for the one or more mobile stations; determininga length of the feedback for each of the one or more mobile stations;and wherein the feedback schedule comprises time slots for each of theone or more mobile stations based on the length of feedback for the oneor more mobile stations.
 3. The method of claim 1, wherein the length ofthe feedback for a first mobile station is determined based on themodulation and coding scheme to be used, the feedback type, the feedbackmatrix dimension, and the number of downlink streams allocated to thefirst mobile station.
 4. The method of claim 1, wherein the feedbacktype includes one of un-compressed channel state information feedback,un-compressed precoding matrix, and compressed precoding matrixfeedback.
 5. The method of claim 4, wherein receiving feedbackcapability includes receiving information regarding which of theun-compressed channel state information feedback, un-compressedprecoding matrix, and compressed precoding matrix feedback are supportedby a mobile station.
 6. The method of claim 1, comprising: setting themodulation and coding scheme for the first frame using the feedbackreceived from the one or more mobile station; transmitting the firstframe to the one or more mobile stations; and receiving refined feedbackfrom the one or more mobile station based on reception of the firstframe.
 7. The method of claim 1, updating the modulation and codingscheme for the one or more mobile stations based on the refinedfeedback.
 8. The method of claim 1, wherein feedback type is assignedbased on a precoding complexity used by the access point, wherein lesscomplex precoding corresponds to feedback types providing lessinformation.
 9. A method for generating feedback at a first mobilestation in a multiple user multiple input multiple output (MU-MIMO)system, the method comprising: receiving a sounding package from anaccess point; calculating a number of streams for other mobile stationsto be transmitted in a downlink frame; determining a feedback matrix forstreams intended for the first mobile station based on the number ofstreams and a reception quality of the sounding package, wherein afeedback matrix type and dimension is specified in the sounding package;and sending the feedback matrix to the access point.
 10. The method ofclaim 9, comprising: receiving an indication of the number of streamsallocated to each mobile station in the downlink frame, whereincalculating a number includes summing the number of streams allocated toother mobile stations.
 11. The method of claim 9, wherein determining amodulation and coding scheme includes calculating the modulation andcoding scheme by applying random and uniform interference from streamsallocated to other mobile stations, and by assuming that streamsallocated to other mobile stations are transmitted with substantiallyequal power.
 12. The method of claim 9, comprising: receiving thedownlink frame from the access point, wherein the downlink frame istransmitted using the modulation and coding scheme sent to the accesspoint; and adjusting the modulation and coding scheme based on thereception quality of the downlink frame.
 13. The method of claim 12,comprising: sending the adjusted modulation and coding scheme to theaccess point on as a piggyback with an ACK/NACK corresponding to thedownlink frame.
 14. The method of claim 9, wherein the feedback is sentto the access point according to a feedback schedule provided by theaccess point.
 15. An system comprising: an access point configured to:announce one or more mobile stations that are to receive downlinkinformation in a frame; send a sounding package to the one or moremobile stations; assign a feedback schedule for the one or more mobilestations; and receive feedback from the one or more mobile stationsaccording to the feedback schedule, the feedback based on reception ofthe sounding package.
 16. The system of claim 15, wherein the accesspoint is configured to: receive feedback capability from the one or moremobile stations; assign a feedback matrix dimension for the one or moremobile stations; assign a modulation and coding scheme for the one ormore mobile stations; assign a feedback type for the one or more mobilestations based on the feedback capability for the one or more mobilestations; determine a length of the feedback for each of the one or moremobile stations; and wherein the feedback schedule comprises time slotsfor each of the one or more mobile stations based on the length offeedback for the one or more mobile stations.
 17. The system of claim15, wherein the one or more mobile stations are configure to: receive asounding package from the access point; calculate a number of streamsfor other mobile stations to be transmitted in the frame; determine afeedback matrix based on the total number of streams and a receptionquality of the sounding package, wherein the feedback matrix isgenerated according to the feedback type assigned in the soundingpackage; and send the feedback matrix to the access point in a time slotassigned in the sounding package and using a modulation and codingscheme assigned by the sounding package.
 18. The system of claim 17,wherein the access point is configured to: adjust precoding matrixesaccording to the feedback received from the one or more mobile stations;and transmit the frame to the one or more mobile stations using theprecoding matrixes.
 19. The system of claim 18, wherein the one or moremobile stations are configured to: receive the frame; refine amodulation and coding scheme based on the reception quality of theframe; and send the modulation and coding scheme to the access pointwith an ACK/NACK corresponding to the frame.
 20. The system of claim 15,wherein the access point is configured to communicate according to anIEEE 802.11ac standard.